The present invention relates to an extrusion molding tool for the production of a hard metal or ceramic drill blank (green blank) made from a plastic raw material and including at least one interior helical rinsing bore, the tool including a shaping die and a mandrel disposed in the intake region of the die. Outside of the longitudinal axis of the die, the mandrel is provided with at least one elastically deformable wire which projects into the die. The shaping cross section of the die includes an inwardly oriented structure which extends helically around the longitudinal axis of the die so as to twist the passing raw material to form one or more helical grooves therein.
Such an extrusion molding tool is disclosed in German Utility Model Patent No. 85/36,805. It is based on the idea that the twisting and the production of the rinsing bore can be performed during formation of the drill blank in one process step together with the extrusion molding. In the known twisting process, the elastic wires provided at the mandrel of the extrusion molding tool produce rinsing bores corresponding to the wire diameters and the uniform geometry of these bores is not destroyed by later process steps. Because of the fact that the blank is twisted by means of helically extending guide projections simultaneously with the extrusion molding process, the twisting no longer depends on external parameters such as the material flow rate in the extrusion press. Independently of the speed with which the punch presses the material into and through the die, the pitch in the hard metal or ceramic drill blank will always be uniform. The twisted drill blank can then be brought to the desired length by a severing process.
Corresponding to the number of desired rising bores, the mandrel of the extrusion molding tool is equipped with one or several wires made of an elastically deformable material. This material should be such that each wire is able to adapt itself to the screwing movement of the raw material in the region of the die with the least possible resistance to deformation and thus to cause the helical rinsing bore to be produced in the plastic raw material. Copper alloys or nonferrous metal alloys, iron, iron alloys, light metals and light metal alloys, particularly those including aluminum, are suitable as the elastic wire material. The use of plastics, such as polyamide, has also been found to be particularly advantageous, also as a coating for otherwise metal wires. With the mutual association of wire and projection in the region of the die, the position of the resulting rinsing bore relative to the helical groove or grooves formed in the drill can be determined. For example, in an embodiment having two grooves and two interior rinsing bores, the angle between the respective connecting lines may be 0.degree. or 90.degree. or take on any desired intermediate values. According to previous experiences, the helix angle of the helical projection in the die should be selected somewhat larger than the helix angle desired for the drill blank grooves; generally the overdimension lies at about 3.degree. to 7.degree..
It has also been found to be advisable to select the length of the die and of the wires to be at least long enough that the helical projection in the die describes a helix of more than 90.degree., preferably more than 180.degree..
German Patent Application Pat. No. 3,714,479.0 discloses arranging the die and the mandrel so as to be fixable in various positions relative to one another in order to be able to inexpensively vary the mutual association of the mentioned components, which influence the formation of the drill blank to be produced, if need be during the extrusion molding process. In such an extrusion molding tool it is possible, on the one hand, to rotate the die and the mandrel relative to one another if the helical rinsing bore is to take on a different angular position relative to the chip (shaving) space of the extruded drill blank. On the other hand, both components can also be longitudinally displaced relative to one another, thus imparting either a greater or lesser deformation to the wire in the direction toward the longitudinal axis of the die and thus making it possible to vary the distance between the rinsing bore and the longitudinal axis of the die.
However, prior art extrusion molding tools have the drawback that selection of the die determines the shape of the helix and the drill type. To be able to work with the same extrusion molding tool, it is necessary to change the die to produce blanks having a different groove pitch or a different diameter, thus requiring the extrusion molding tool to be disassembled from its associated holding structure. The work involved with this is complicated and time consuming.
Additionally, there is a desire to manufacture drills of different diameters, for example from 3 mm to 6 mm, with one and the same extrusion molding tool and the respective drill is to be produced from a blank having a diameter of 6 mm. In this case, all drills, although they have different diameters, should have the same pitch, for example a pitch of 30.degree.. Since the length of the path over which a helix having a certain pitch, e.g. 30.degree., describes a full circle (360.degree.) in a 6 mm drill differs from the corresponding path in a 3 mm drill likewise having a 30.degree. helix, a 6 mm drill blank cannot be reduced in diameter to 3 mm simply by grinding material away since this would inevitably cut into the rinsing bore helixes. In other words, the rinsing bore would be exposed on the drill circumference. For this reason as well, it is necessary to install a different die in the extrusion molding tool for each drill diameter.